EP-A 0 325 262 and EP-A 0 314 317 disclose corresponding fusion proteins composed of various domains of the CD4 membrane protein of human T cells and of human IgG1 portions. Some of these fusion proteins bind with the same affinity to the glycoprotein gp120 of human immunodeficiency virus as the cell-bound CD4 molecule. The CD4 molecule belongs to the immunoglobulin family and, consequently, has a very similar tertiary structure to that of immunoglobulin molecules. This also applies to the α chain of the T-cell antigen receptor, for which such fusions have also been described (Gascoigne et al., Proc. Natl. Acad. Sci. USA, vol. 84 (1987), 2937–2940). Hence, on the basis of the very similar domain structure, in this case retention of the biological activity of the two fusion partners in the fusion protein was to be expected.